Balancing the effects of rearing at low temperature during early development on sex ratios, growth and maturation in the European sea bass (Dicentrarchus labrax). Limitations and opportunities for the production of highly female-biased stocks

Article in press.In many cultured fish species, including the European sea bass, undesirable highly male-biased sex ratios are nevertheless frequent. High temperatures (~ 21 °C), typically used during the larval and early juvenile stages are thought to cause sex-reversal of genotypic females. Rearing at lower temperatures (<17 °C) has been investigated as a possible solution. However, besides reducing growth, they have sometimes resulted in equivocal sex ratios across different studies. The goal of this study was to find a thermal regime that could maximize the number of females without compromising growth. Four batches were reared at 15 °C during 10 (control group), 30, 60, 90 or 120 (treated groups) days post fertilization (dpf) and then at 21 °C. The controls had excess males (average from 4 batches: 69% males, 31% females). Increasing the duration of rearing at 15 °C doubled the average number of females up to 59% (range 22–90%) and reduced the number of 1-year-old precocious males from 29% in the control to 10–20% in the treated groups. Low temperature retarded growth in groups exposed for 60 or more days; however, fish exposed for 30 days exhibited compensatory growth by 150 dpf. Females reached marketable size (set at 400 g) during the second year, four months earlier than males. Despite initial slower growth, an increase of 10% in biomass was estimated after rearing at 15 °C for ~ 60 days (850–900 °C-days) by the time of marketing when compared to current industry practices. Finally, we analyzed these and all available data of the effects of temperature on European sea bass sex ratios, and show that exposures to low temperature starting at fertilization do not induce but merely allow female development. In contrast, high temperatures masculinize, on average, over half of the females. The results show that insensitive fish are not males that cannot be feminized by low temperature but instead females that cannot be masculinized by high temperature. Although there is no thermal regime that per se will result in 100% females in the European sea bass, we propose rearing at 17 °C starting soon after fertilization and until 850–900 °C-days, which at that temperature occurs at ~ 53 dpf, as the best balance between the advantages resulting from allowing female development, and the disadvantages of initial slower growth. Combined with genetic selection for high female number and low sensitivity to high temperature, this method offers an opportunity for the routine culture of highly female-biased sex ratios to benefit European sea bass aquaculture.Peer reviewe